This invention relates in general to a process for setting bearings and more particularly to a process which accounts for dimensional deviations without actually assembling the bearing.
Two tapered roller bearings when mounted in opposition produce a bearing arrangement that is capable of accommodating heavy radial loads as well as thrust loads in both axial directions. As a consequence this bearing arrangement finds wide spread use in automotive applications. For example, the nondriven wheels of automobiles and over-the-road trucks are usually mounted on spindles or axles by means of this arrangement. Usually the cup of each bearing is pressed into the hub of the wheel, while the cone assemblies (the cone, the tapered rollers, and the cage which holds the rollers around the cone) are fitted over the spindle or axle, whatever the case may be, such that the small diameter ends of the rollers are presented inwardly. This is known as an indirect mounting. The axial positions of the two cups and the two cones are critical for those positions determine the setting of the bearing arrangement. The setting may be one of end play in which the hub can move axially a short distance relative to the spindle or axle on which it is mounted, or it may be one of preload in which no axial play exists and the tapered rollers are actually compressed between their respective cups and cones for the full 360.degree. of the two bearings. Excessive preload is undesirable since high friction develops in the bearings and substantial stresses are imposed on their raceways. As a result bearings adjuted to a condition of high preload fail much sooner than bearings adjusted to moderate preload or slight end play. Also a condition of high preload leaves the bearing arrangement with little or no capacity for accommodating thermal expansion. On the other hand, a condition of excessive end play does not provide the stability required, for it permits considerable wobbling of the wheel. It also lessens the life of each bearing.
In automobiles and most over-the-road trucks the bearing setting is adjusted quite easily and with adequate precision, merely by turning a nut that is threaded over the spindle or axle. This nut serves as a backing for the outboard cone and hence controls the spacing between the two cones. During the adjustment, the wheel is easily turned to insure that the tapered rollers of the two bearings seat against the raceways of their respective cups and cones and against the thrust ribs of the cones.
The typical arrangement of two indirectly mounted tapered roller bearings is also used in much larger equipment where adjustment procedures are not nearly so simple. For example, the wheels of large off-highway vehicles such as the trucks used at quarries and at strip coal mines, as well as the wheels on large excavating and mining equipment, are mounted with bearings that can exceed 30 inches in diameter. A single wheel and its mounting may weigh as much as 12,000 pounds.
The present procedure for installing a large wheel on a stub axle commences with orienting the stub axle in an upright position with its outboard end presented upwardly. The inboard cone is then heated to expand it enough to fit over the axle, and the entire cone assembly is moved downwardly over the stub axle until the back face of the cone comes against a shoulder. Also, the two cups are pressed into the wheel hub. Next the hub is lowered over the axle until the lower or inboard cup as well as the entire wheel is supported entirely on the lower or inboard cone assembly. The rollers of the lower bearing in this instance seat easily against the tapered raceways of the cone and cup, and the large diameter ends of the tapered rollers bear against the cone thrust rib, having originally dropped to that position by virtue of their own weight. Thus, no end play exists in the lower bearing at this step in the assembly procedure. Thereupon, the cone of the upper or outboard cone assembly is heated and that assembly is dropped onto the stub axle until its rollers come against the tapered raceway of the outer cup. The large diameter ends of the rollers may or may not seat against the cone thrust rib, but most likely they will not. Next, the individual assembling the mounting selects a shim pack, and the thickness of this pack is determined solely by his experience. Once the shim pack is in place, an end plate is installed on the outer end of the axle, and it serves as a backing for the outer cone. The end plate is held in place by a large number of bolts.
Installation of the end plate does not complete the assembly procedure, for the mounting must be checked to see if the bearings have the correct setting. This is achieved with dial indicators positioned to measure the relative movement between the hub and stub axle. With the dial indicators in place, the hub is attached to an overhead crane and lifted upwardly until the stub axle is suspended completely from the hub. The assembler then observes the dial indicators to determine if any change has occurred. Theoretically, the change in the measurement made by the dial indicators represents the end play in the bearing assembly. Often the measured end play does not fall within prescribed limits, in which case the end plate is removed and the shims are changed. After the end plate is replaced, the end play is again measured by lifting the hub with a crane. This procedure for measuring may be repeated several times. The foregoing procedure is not only time consuming, but it is also not very precise and can lead to errors of a substantial magnitude. First, the dial indicators do not actually reflect end play, but instead they meausre end play plus deflection of the bearing and its related mounting components. In this regard, the bearing and mounting components are not entirely rigid and will deflect, particularly when subjected to heavy loads which are clearly present during the measuring procedure. Secondly, the tapered rollers of the outer bearing may not seat against the cone thrust rib, and indeed gravity urges them away from the thrust rib. Any spacing between the large diameter ends of the rollers and the cone thrust rib represents end play which is not measured. Third, the assembler may initially select a shim pack that places the bearing under severe preload, but this preload may incorrectly appear as end play due to deflection in the bearing and its mounting components. As a result the mounting may be released with its bearings set in a condition of excessive preload. Finally, even if such excessive preload is recognized before release of the mounting, the interference fit between the axle and the cone of the outer bearing presents additional complications. The assembler may simply replaces the initial shim pack with the correct shim pack, in which case the cone may control the spacing instead of the shim pack. Alternately the assembler may properly relieve the excessive preload by pressing the outer cone outwardly a slight distance before applying the corrected shim pack.